1. Field of the Invention
The present invention is in the field of plant transformation, especially as it pertains to Zea mays. More specifically, the invention relates to methods and compositions for the production of plant varieties with improved transformability.
2. Description of the Related Art
In addition to breeding, the ability to culture and regenerate plants via tissue culture techniques has been useful in the study and advancement of plant manipulations. Beneficial alterations in the genome may occur during the culturing and regeneration of a plant such that new or improved agronomic traits result. Culturability of a given crop plant appears to vary with the germplasm used, with some lines being easier to culture and regenerate than others. In many instances, however, plants with superior agronomic traits tend to exhibit poor culturing and regeneration characteristics while plants that are more easily cultured and regenerated are often agronomically poor.
In corn, for example, agronomically desirable lines such as B73 or FBLL show reduced culturability and regenerability relative to agronomically poor, yet culturable lines such as A188 or H99. It is the experience of the present inventors that a genotype may be culturable but not transformable. For example, immature embryos were isolated from a number of elite corn lines and tested for culturability and transformability; while a number of lines formed callus, most of the lines were not transformable under the conditions employed.
Work by Armstrong and others (1991, 1992) showed that it was possible to interbreed a more culturable, agronomically poor maize line (A188) with an agronomically desirable, less transformable line (B73) to produce a novel line with increased culturability and regeneration (Hi-II). Marker analysis of the line was carried out and identified several chromosomal regions that appeared to confer increased culturability on the less culturable genetic background. More specifically, Armstrong and others (1992) identified markers on chromosomes 1, 2, 3, and 9 as being associated with enhanced culturability and regenerability. A marker on chromosome 9, c595, was reported to be particularly relevant for the formation of callus in the new germplasm as well as the ability to regenerate plants.
Breeding is a traditional and effective means of transferring the traits of one plant to another plant. Marker assisted breeding is a means of enhancing traditional breeding and allowing for selection of biochemical, yield or other less visible traits during the breeding process. Transformation is a new molecular technique that is effective in transferring DNA from a variety of sources into a plant in order to obtain plants with improved agronomic or novel traits. While breeding work has been carried out to improve plant culture and regeneration, virtually no research has been carried out to identify and breed for chromosomal regions that are linked with enhanced transformation characteristics.
Knowledge of the markers, chromosomal regions and genes that result in increased transformability would be beneficial to the art of plant transformation. Transformability of plants such as corn plants often varies with the germplasm, with some lines being more transformable than other lines. Typically, more transformable lines are typically agronomically poor (for example Hi-II) while lines with superior or desired agronomic traits are less transformable (for example FBLL). If a desired gene is introduced into an agronomically poor line, it is then commonly introgressed into an elite or superior line for testing such parameters as efficacy of the introduced gene as well as to test the effect of the gene on such traits as yield, kernel quality and plant phenotype. Thus, to enable meaningful performance testing in earlier generations, it would be advantageous to be able to introduce the genetic components resulting in enhanced transformability into agronomically superior lines.
The present invention overcomes this deficiency in the art by providing a method of breeding for increased transformability and resultant maize plants.